Zanshin is a Python library for collaboration over HTTP, WebDAV and CalDAV. It was originally conceived by Lisa Dusseault, and is currently being developed and maintained by Grant Baillie. Its primary client is the Chandler Sharing Project.
* High-level API: Zanshin works at the level of resources and properties of resources, rather than HTTP requests and responses. This, coupled with careful thought about what data to persist, will hopefully lead to an easier-to-use and better performing API than what you get by making the obvious 1:1 mapping between Python method calls and HTTP requests.
* Asynchronicity via the Twisted networking framework. For an excellent discussion of Chandler's use of Twisted, see TwistedHome.